Architecture revision

[one000mph]

Overview of Changes

• Updates TOC/sidebar to reflect new Outline (Issue Implement a functional prototype #2)
• Condensed Architecture Guide
• Auth Service
  • Authn vs Authz
  • Purpose of Certificates
  • Issuing & Using Certificates
  • Certificate Rotation
  • Auth Server API
  • Auth State
• Node Service
• Proxy Service
• Users
• A few typo corrections in code comments (let me know if you prefer that I don't correct these)

Motivation
This PR introduces the Architecture rather than the Architecture Guide. This change attempts to address a couple of issue I saw in the previous docs version.

1. Many of the chapters were quite long (2000+ words). Even with the sidebar TOC to help users jump to sections within pages, some pages felt difficult to navigate and digest.
2. The same idea (e.g. Auth Service) was explained in multiple places, but there was not one source definition or description to point users to.

In this PR I attempted to pull out the key concepts of Teleport and write explainer docs on each of them. I also attempted to eliminate the need for repetitive explanations of concepts in the step-by-step guide. Guides can now focus on practical tasks and provide minimal inline definitions. They will reference users to concept docs for detailed explanations of terms and ideas.

Possible Problems
As I was writing I found that it was hard to describe each concept alone without duplicating some information between docs. For example the Proxy Service relies heavily on the Auth Service so you can't really discuss one without the other. As these are refined it will be challenging to describe each concept clearly while minimizing duplicate information.

[gravitational-jenkins]

Can one of the admins verify this patch?

[benarent]

@one000mph This PR still has a few merge conflicts. Could you resolve these please.

[one000mph]

@benarent yep just pushed.

[benarent]

Approving the PR, as per the last PR I'm going to squash / merge. Then perform a cleanup review.